In recent years, wireless (radio) functions have been added to miniature sensors for transmitting information detected by the sensors by wireless communication, and sensor networks formed by connecting these sensors to a network have become the subject of much attention. Such wireless communication function equipped sensor includes a wireless communication function and power supply (battery) inside the sensor unit. Since it has neither wires nor power supply terminals, it can easily be installed in any locations where it was until now impossible to install the sensors.
The wireless communication function equipped sensor uses a battery as a power supply, but it can be provided with a generator for generating electricity from the vibration, light or heat depending on the installation locations to be capable of semi-permanent operation.
Sensor information detected by the wireless communication function equipped sensor is transmitted intermittently at fixed intervals and electric power consumption can be reduced by operating the wireless communication function equipped sensor at intervals. In other words, the wireless communication function equipped sensor is provided with a generator having a small capacity of power supply to reduce the size of sensor. The wireless communication function equipped sensor utilizes such a method that it transmits sensor information for a fixed period of time, when the electric power generated by the generator reaches a level sufficient to drive the wireless communication function equipped sensor. By using this method, even if a generator has a small capacity of power supply, it can transmit sensor information.
An example of sensor which can periodically transmit sensor information is disclosed by Patent document 1. According to Patent document 1, an impeller (shaft with vanes) is installed in water passage and is linked to a generator, which converts the rotational force of the impeller into electric power. The use of water supply is detected by the electric power output from the generator, and an electrical circuit transmits that information by wireless communication.
[Patent document 1] JP-A No. 287818/1999 (See page 2 through page 3, FIG. 1 and FIG. 2)
The prior art technology has proposed a method for transmitting sensor information detected by the wireless communication function equipped sensor wirelessly to communication destination. However, the prior art technology gives no consideration for installing a function to receive information from the communication destination. In other words, there has been a need to provide the wireless communication function equipped sensor with a function for receiving information from the wireless host when changes in the operating mode, installation settings, or program changes were made.
When a function for receiving information is added to the wireless communication function equipped sensor, it is necessary that the receiving circuit is always in standby state to receive information from the wireless host at any time.
However, if a wireless communication function equipped sensor using an internal battery is always in standby state, it will shorten the service life of the battery to cause the troublesome task of changing the battery. Also if the wireless communication function equipped sensor has an internal generator, it is difficult for the sensor to be always in standby state since such generator has only a small capacity of power supply.